Bake for 20 minutes in the preheated oven, or until toothpick inserted in center of cake comes out clean.

While cake bakes, prepare frosting. Combine 1/2 cup butter or margarine, milk and 2 tablespoons cocoa in a saucepan. Heat to boiling, stirring constantly. Beat in confectioners' sugar and vanilla until smooth. Spread warm frosting over cake as soon as it is taken from the oven. Cool.